Influence of ionic strength and carbonate on the Ca-P coating formation from SBF×5 solution

Biomimetic calcium-phosphate (Ca-P) coatings were applied on Ti6Al4V by using simulated body fluids concentrated by a factor 5 (SBF×5). The production of SBF×5 solution was possible by decreasing the pH of the solution to approximately 6 using CO2 gas. The subsequent release of this mildly acidic gas led to a pH rise and thus, increasing supersaturation. After immersion for h a Ca-P coating on Ti6Al4V plates and a precipitate simultaneously formed at pH=6.8. Sodium chloride (NaCl) and hydrogencarbonate (HCO3−) contents were studied in relation to CO2 release and coating formation by changing their individual concentration in SBF×5 solution. On one hand, NaCl-free or low NaCl-content SBF×5 solution led to the earlier aspecific precipitation in the solution than for SBF×5 solution. In contrast, Ca-P coating was formed later and was thinner than the coating obtained in regular SBF×5 solution. High ionic strength delayed precipitation and favored Ca-P heterogeneous nucleation on Ti6Al4V. On the other hand, HCO3− content increased the pH of the solution due to its buffering capacity and influenced the release rate of dissolved CO2. Thus, HCO3− content strongly affected the supersaturation and Ca-P structure. Furthermore, HCO3− favored the attachment of Ca-P mineral on Ti6Al4V by decreasing Ca-P crystal size resulting in a better physical attachment of Ca-P coating on Ti6Al4V substrate.